PostgresPlatform's modify_column_string missing {length}?

[os-nikita]

Describe the bug
It looks like for Postgres, using for example table.string("foo", 36).change() in a migration results in the column taking on a varchar field without any length specified. This seems to be due to the what is mentioned in this issue's title.

Expected behavior
Migrating a table to change a column should add the appropriate string length. Compare this to MySQLPlatform.py, for example.